That means it probably formed around a star and was somehow shot out of its orbit into lonely deep space (see artist’s impression, above). That also makes this planet much easier to study than those that are almost lost in the dazzle from the stars they circle.

“You have to work really hard to even see them, whereas this object is just by itself,” says Beth Biller at the University of Edinburgh, UK.

Advertisement

Biller’s team measured the planet’s brightness and found that it could vary by up to 10 per cent in just a few hours. The explanation, they say, could lie in its weather systems.

Stormy spots

“If you think about the Great Red Spot on Jupiter, it would be stormy spots like that,” Biller says. Both worlds have similar rotation periods: 10 hours for Jupiter, and between 5 and 10 hours for the lone planet.

But unlike Jupiter, which has cooled from a hot start over the long life of our solar system, this planet retains a scorching surface temperature of about 1100 kelvin – maintained by internal heat since it has no star.

Those conditions mean that any clouds it has should be molten, containing liquid metals where on Earth we would have water. “These are likely hot silicates and iron droplet clouds,” Biller says. “This makes Venus look like a nice place.”

Caroline Morley, who models exoplanet atmospheres at the University of California, Santa Cruz, thinks the finding may mean that similar planets – whether orbiting stars or not – might show the same behaviour.

“It strongly suggests that these objects should be variable [in brightness],” Morley says. “We really want to be able to look at this variability and then connect it to storm systems.”

Biller’s team is already trying to tease out a similar analysis from observations of a star called HR 8799, which has planets closely resembling this lone world.

That makes this an exciting time to be looking for exo-weather, Morley says. “People have been saying for years that we [should] look at brown dwarfs because they’re like planets, and we can make these connections,” she says. “But now we’re actually starting to do that.”